1. Field of the Invention
The diminishing reserves of petroleum oil have focused attention on the need to find alternative sources of carbonaceous materials and stimulated considerable interest in the possibility of making more effective use of the world's vast reserves of natural gas. At the present time, only a minor fraction of the available methane is being utilized. In the U.K., for example, it is used both as a fuel and as a feedstock, via steam reforming to synthesis gas (carbon monoxide and hydrogen), for methanol and ammonia synthesis, but in many parts of the world the collection and distribution of methane are uneconomical and it is burnt in situ to form carbon dioxide and water.
There are several known reactions for the oxygenation of methane. EQU CH.sub.4 +O.sub.2 .fwdarw.CH.sub.3 OH EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+H.sub.2 EQU CH.sub.4 +O.sub.2 .fwdarw.CH.sub.2 O+H.sub.2 O EQU CH.sub.4 +O.sub.2 .fwdarw.C.sub.2 H.sub.4 +C.sub.2 H.sub.6 +CO.sub.2 +CO+H.sub.2 O EQU CH.sub.4 +O.sub.2 .fwdarw.CO.sub.2 +H.sub.2 O
Different catalysts promote these reactions to different extents, but selectivity is normally poor. This patent application results from our discovery of a class of catalysts that is capable of selectively oxygenating methane to carbon monoxide and hydrogen.
2. Description of the Prior Art
Recently, attempts to convert methane directly into more valuable chemicals have focused on oxidative coupling reactions to yield ethylene and ethane: Keller, G. E. & Bhasin, M. M., J. Catal. 73, 9-19 (1982); Hutchings, G. J., Scurell M. S. & Woodhouse, J. R., Chem. Soc. Rev. 18, 251-283 (1989); and Ashcroft, A. T., Cheetham, A. K., Green, M. L. H., Grey, C. P. & Vernon, P. D. F., J. Chem. Soc. Chem. Commun. 21, 1667-1669 (1989), and direct oxygenation to methanol and formaldehyde: Gesser, H. D., Hunter, N. R. & Prakash, C. B., Chem. Rev. 85, 235-244 (1985); and Spencer, N. D. & Pereira, C. J., J. Catal. 116, 399-406 (1989). Unfortunately, under conditions where the reactions of methane are fast enough to be of interest (typically&gt;700.degree. C.), the formation of CO.sub.2 is so favorable (.DELTA.G.rarw.800 kJ/mol) that partial oxidation to more useful products is difficult to achieve on an economical scale. The non-catalytic, gas-phase partial oxidation of methane to synthesis is an established industrial process (e.g., Shell, Texaco: "Encyclopedia of Chemical Technology", Ed. Kirk. R. E. and Othmer, D. F. 3rd Edition, Wiley Interscience, N.Y., Vol. 12, 952 (1980)), but operates at very high temperatures (&gt;1200.degree. C.). Synthesis gas mixtures are also formed in two step catalyzed reactions using mixtures of methane, water and oxygen which operate at elevated pressures and temperatures in excess of 1000.degree. C., Encyclopedia of Chemical Technology, lbid. This patent application results from our discovery of catalysts that are capable of selectively oxygenating methane to carbon monoxide and hydrogen so that the reaction can be carried out catalytically and at a substantially lower temperature (.about.775.degree. C.). The significance of this result lies in the fact that synthesis gas is a well established feedstock for the synthesis of higher hydrocarbons, alcohols and aldehydes, for example in Fischer-Tropsch catalysis, for example, Henrici-Olive, G. & Olive, S., Angew. Chem. Int. Ed. Eng. 15, 136-141 (1976), thus facilitating efficient two-step processes for the conversion of methane to such materials. Equally, one possible application for synthesis gas produced at low pressures, is for use in fuel cell technology.
The overall reaction which is catalyzed is: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.CO+2H.sub.2
and this reaction is often described as the partial oxidation of methane.
As noted above, synthesis gas can be made by a number of methods, most of which involve the steam reforming of hydrocarbons or coal, "Catalysis in C.sub.1 Chemistry", Ed. Keim., W., D. Reidel Publ. Co., Dordrecht, (1983). Methane, for example, can be converted over a nickel/alumina catalyst, Rostrup-Nielsen, J. R. in "Catalysis, Science & Technology, Vol. 5" (ed. Anderson, J. R. & Boudart, M., Springer, Berlin (1984) and Topp-Jorgensen, J., in "Methane Conversion" (ed. Bibby, D. M., Chang. C. D., Howe, R. F., and Yurchak, S.) Elsevier, p.293(1988), at 700.degree.-800.degree. C., according to: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO=H.sub.2
This reaction is an important source of carbon monoxide and hydrogen, but it is highly endothermic, and leads in addition to the formation of carbon dioxide via the water-gas shift equilibrium: CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2. The partial oxidation reaction, by contrast, is mildly exothermic, more selective, and yields an H.sub.2 /CO ratio that is lower than that obtained by steam reforming. This lower ratio may be highly desirable for certain applications of synthesis gas. Indeed, secondary reformers using CO.sub.2 or O.sub.2 oxidants are frequently required to reduce the hydrogen content of synthesis gas made by steam reforming.
In FR 1595993, Chimigaz, there is described a method for the catalytic partial oxidation of methane to carbon monoxide plus hydrogen But the temperatures of 1000.degree.-1200.degree. C. were so high as to be uneconomic.
In EPA 303 438, Davy McKee Corporation, there is described a catalytic partial oxidation process for converting a hydrocarbon feedstock to synthesis gas. The process described uses steam in addition to oxygen and runs at temperatures of 870.degree. C. to 1040.degree. C. and a pressure of about 2760 kPa. Even under optimum conditions, conversion of methane to a product consisting essentially of hydrogen plus carbon monoxide in the substantial absence of steam and carbon dioxide is not achieved, i.e. the (H.sub.2 +CO) selectivity of the system is not very good. At lower temperatures and pressures, particularly when using low concentrations of steam, methane conversion and (H.sub.2 +CO) selectivity fall off and the catalyst becomes poisoned by carbon deposition.